In ultrasound imaging modes such as elasticity, strain, color-flow, and B-flow wherein tissue or flow motion needs to be detected, multiple firings of beams are performed to extract motion information from received signals that typically include a relatively high signal-to-noise ratio. In such modes, a minimum time interval is required between repeated firings of the same beam to allow sufficient displacement of tissue or flow being measured. When the motion or displacement to be measured occurs relatively slowly, more time is needed between firings to obtain a measurable difference in received echo phase. This minimum motion or displacement time is often more than a multiple of the minimum time needed to receive echoes from a maximum image depth, and/or the minimum time needed so that echoes from a first firing are attenuated sufficiently so that these echoes will not interfere with echoes from a second firing. Accordingly, for an example minimum motion or displacement time interval of 1 unit, a pulse repetition interval (PRI) should be greater than 1.
Typically, a minimum motion or displacement time interval may be so long that a frame rate of acquired images may become undesirably long, especially if transmitting all the firings of one beam consecutively. Accordingly, interleaving of beam firings is typically used to increase a frame rate of acquired images. Such interleaving is typically accomplished by firing multiple different beams within a time interval, such as a PRI, between two consecutive firings of at least one of the beams.
In conventional ultrasound elasticity, color flow, or B-flow imaging techniques, interleaving groups using multiple beam firings are commonly used. FIG. 1 depicts a prior art scanning sequence 10 using interleaving groups. The beams used in the scanning sequence 10 are represented by a letter designation 16, such as A, B, C . . . etc. Beam firings are designated by a firing index number 18, such as 1, 2, 3 . . . etc. A firing sequence 20 is indicated by arrow 22 traversing a set of firing indices and a number of the sequence is indicated by a superscript 24.
If a PRI between firings of beam A is desired to be a certain amount (e.g., a PRI of five) so that N−1 beams (e.g., four) can be inserted in between, typical practice is to group N beams into one group 12, transmit the first firings (indicted by the firing index 1 24 for each of the beams 12) of the N beams sequentially during the first PRI, and then, during a next PRI, transmit the second firings of the N beams, etc., until all the firings are transmitted. As indicted in FIG. 1, the first firings of beams A-E are sequentially transmitted, followed by the second firings of beams A-E, etc., until a desired number of firings of each of the beams is reached (such as four firings, as indicated in FIG. 1). Then, another group 14 of beams is transmitted in the same manner until the desired number of firings is reached. However, this technique generates beam interleaving discontinuities between groups, causing formation of artifacts in a resulting image. Such artifacts may be especially pronounced in tissue elasticity imaging when interleaving groups are used.